Tp prepare for my first trip in years and years I did several things, all of which really helped.
1) Buy the Unofficial Guide to Walt Disney World. Get the newest edition possible (not an old one from the library, it will be out of date) and read it.
2) Subscribe to touringplans.com. They are the companion site to the Unofficial Guide. You get a discount if you own the book. They have a crowd calendar (best, least crowded park to visit each day) and additional touring plans in addition to what is in the back of the book.
3) Follow a touring plan in the parks. This will make sure you see everything in the most efficient way possible (less waiting in line). A lot has changed in the past 10 years and you'll want to see/do it all!
4) If you want to eat at any sit down restaurants on Disney property make reservations (called ADRs). You can make them 180 days in advance and for the very popular places you NEED to make them 180 days in advance.
5) Search the Dis for Disney discounts (room only discounts, free dining discounts, etc), for extra special experiences (Mickey's Not So Scary Halloween party, Bibbidi Bobbidi Boutique. Pirate's League, Jedi Training Academy, etc). Search and or post whenever you have a question or need some advice about anything Disney.
6) Have a GREAT time!!!!!